The proposed work is directed at studying two phenomena: (1) The mechanism of peroxidase catalyzed thyroxine synthesis from free, polypeptide, and protein bound iodotyrosine precursors. (2) To measure the rate and extent of hydrogen peroxide production in the thyroid gland $ith cytochrome c peroxidase. BIBLIOGRAPHIC REFERENCES: Alexander, N.M. and Benson, G.D., Superoxide dismutase activity (erythrocuprein) in Wilson's Disease, Life Sciences 16, 1025-1032 (1975). Alexander, N.M. and Jennings, J.F., Analysis for total serum thyroxine by equilibrium competitive protein binding on small, re-usable Sephadex columns, in "Clinical Radioassay Procedures: A Compendium" (Amer. Assoc. Clin. Chem.) 271-277 (1975).